Manhattan prosecutors yesterday admitted that they failed to present a key piece of evidence to grand jurors in the case of two cops accused of raping a drunken female clubgoer they had escorted home.

The embarrassing revelation came as jury selection was about to begin in the case against Officers Kenneth Moreno and Franklin Mata and was met by a stern rebuke from Manhattan Supreme Court Justice Gregory Carro, who was clearly annoyed that prosecutors had waited until the very last minute to admit their blunder and ask for a two-week delay so they could re-present the case to a new grand jury.

Jury selection had been scheduled to begin today, as the cops were set to fight accusations that they “helped” a drunken 27-year-old fashion executive to her East Village apartment last year and that Moreno raped the woman as Mata stood guard.

Instead, prosecutor Randolph Clarke asked the judge to delay the trial until March 21.

“The people are requesting March 21, Your Honor, based upon some evidence we discovered. We are seeking to re-present this to the grand jury,” Clarke told Manhattan Supreme Court Justice Gregory Carro.

“But this is not new evidence,” the judge responded — noting that fault for the delay lies with the prosecution. “This is something you are trying to correct in the grand jury.”

“Yes, Your Honor,” Clarke said.

“I’m not sure why this took so long to figure out,” an annoyed Carro responded.

When the prosecutor promised to get a transcript from the second grand jury to the judge by Friday, March 18, so it could be reviewed in time for the trial to start the following Monday, the judge added, “In light of the fact that you are delaying the case, I’d appreciate it.”

The error was not elaborated on in court by prosecutors.

Moreno’s lawyer, Joseph Tacopina, complained on the record that “this is something that has been in their [prosecutors’] possession for 2½ years.”

Both sides are barred by grand-jury secrecy laws from describing the omitted evidence outside court.

“They simply overlooked a piece of evidence that we believe is very favorable to the defendants in this case,” Tacopina told reporters.

A law-enforcement source insisted the overlooked evidence is helpful to the prosecution and will lengthen the original indictment — though not with any major additions.

The two cops are insisting that no sex whatsoever happened in the woman’s apartment and that Moreno falsely admitted otherwise into his accuser’s hidden microphone because she had repeatedly threatened to march up to Moreno’s Ninth Precinct desk sergeant and “make a scene.”